BEIRUT: Syrian regime forces were on high alert and taking precautionary measures on Tuesday at military positions across the country amid fears of a US strike in the aftermath of an apparent chemical weapons attack near Damascus. Syria on Tuesday invited the international chemical weapons watchdog to send a fact-finding mission to investigate the poison gas attack in a rebel-held town over the weekend. The Foreign Ministry said Syria will help the Organization for the Prohibition of Chemical Weapons to investigate the alleged attack, which opposition activists say killed 40 people. President Donald Trump has threatened military action, vowing to respond “forcefully” to Saturday’s attack on civilians in the town of Douma and warning that Russia — or any other nation found to share responsibility — will “pay a price.” Trump on Tuesday canceled plans to travel to South America later this week, choosing to stay in the US to manage the response to the events in Syria. The decision marks the first time an American president has not attended the Summit of the Americas. Vice President Mike Pence will travel in Trump’s place. UN Secretary-General Antonio Guterres said Tuesday that he was outraged by the reported attack, and that the use of chemical weapons would be a violation of international law. He also reaffirmed his support for an OPCW investigation. A senior Russian lawmaker said Tuesday that Moscow is willing to help arrange an OPCW visit to the site of the suspected attack. Yevgeny Serberennikov, from the defense committee at the Federation Council, told the RIA Novosti news agency that Russia is eager for the OPCW to “finally start carrying out the functions it was created for.” Asked in Berlin whether she supports the idea, German Chancellor Angela Merkel responded unenthusiastically, saying that “the evidence that chemical weapons were used there is very, very clear.” The Britain-based Syrian Observatory for Human Rights said that as of Monday night, troops have been on a 72-hour alert and are fortifying their positions. The observatory’s chief, Rami Abdurrahman, said the alert includes all army positions and bases from the southern province of Sweida all the way to Aleppo province in the north, the Mediterranean coast in the west to Deir Ezzor province in the east, along the Iraqi border. The Sound and Picture Organization, an activist collective in eastern Syria, said that Iranian fighters and members of Lebanon’s Hezbollah group have evacuated their positions in the Boukamal area.